1. Field of the Invention
The present invention relates to a micromachine, such as an optical switching element for use in an image display device, and to a manufacturing method therefor.
2. Description of Related Art
An optical switching element using liquid crystals has been known as that enabled to control the switching-on and switching-off of light. FIG. 13 schematically shows the configuration thereof. This optical switching element 900 may consist of polarizing plates 901 and 908, glass plates 902 and 903, transparent electrodes 904 and 905, and liquid crystals 906 and 907. The direction of alignment of liquid crystal molecules is changed by controlling a voltage applied between the transparent electrodes 904 and 905. Thus, the plane of polarization thereof is turned thereby to perform optical switching. Consequently, an image display device can be constituted as a liquid crystal panel by arranging such optical switching elements (namely, liquid crystal cells) in a two-dimensional form.
However, this optical switching element using liquid crystals is poor in high-speed response, and operates at a response speed of as low as about several millimeters per second. It is, therefore, difficult to use a spatial light modulator, which employs liquid crystals, in optical communication, optical computing, an optical storage device such as a hologram memory, and an optical printer, which require a high response speed. Further, the spatial light modulator employing liquid crystals has a drawback in that efficiency in using light is degraded by the polarizing plates.
This enables higher-speed switching. Thus, optical switching elements, whose efficiency in using light is high, are demanded. One such optical switching element is a spatial light modulator enabled to achieve high-speed modulation by mechanically moving an optical element that can control light. An example of the spatial light modulator made to be highly practical is a micromirror device. This device is adapted so that a mirror serving as an optical element is turnably supported by a yoke, and that incident light is modulated by changing the angle of the mirror, and then outputted therefrom.
Alternatively, incident light can be modulated by moving the position of a planar optical element having a reflection function or a transmission function. Thus, a spatial light modulator adapted to mechanically move such an optical element may be employed. Applicant of the present application assiduously develops one such spatial light modulator adapted to extract evanescent light from a total reflection surface of a light guiding portion, which can transmit light by performing total reflection thereof, by bringing a switching portion, which has an extraction surface adapted to transmit light, (in an on-state) into contact with the total reflection surface. This switching portion is turned off by being detached from the total reflection surface by a minute distance of about one wavelength or less. This spatial light modulator can switch on and off light by moving the switching portion, which functions as an optical element, by a minute distance. Thus, this spatial light modulator is an optical switching element that can modulate and control light at a high speed. Electrostatic force generated by applying a voltage to the electrodes is mainly used as means for driving the switching portion.
In the case of using this spatial light modulator, it is necessary to move the switching portion with good accuracy by a distance on the order of the wavelength, for example, about one-tenth of several microns, namely, a distance of submicron order. Therefore, to manufacture the switching portion and a drive portion, a manufacturing method, by which such portions are manufactured with a sufficient precision of one micron, is demanded. Especially, the switching portion is an optical element, so that accuracy, which is commensurate with or less than the movement distance of submicron order, is demanded as the surface finish accuracy thereof. Furthermore, when a two-dimensional image is actually generated by the spatial light modulator, it is necessary to arrange a plurality of switching elements in an array. Thus, there is the necessity for manufacturing a large number of switching elements of the same constitution with precision of one micron or less in such a way so as to adjoin one another.
Techniques of manufacturing such elements with precision of one micron have currently been being developed. Such techniques are, for instance, photolithography techniques having progressed as techniques for manufacturing semiconductors. According to present design rules, necessary precision is about one-tenth of several microns. When the design rules are met, the precision needed for manufacturing the aforementioned spatial light modulator can be obtained. In addition, there has been developed a manufacturing method of manufacturing a structure with precision of one micron by utilizing manufacturing techniques, such as an oxide film formation technique, a crystal growth technique, a CMP planarization technique, a laser processing technique, a sol-gel forming technique, a sintering technique, and a machine cutting technique. Thus, a machine, which is called a micromachine and has structure of the size represented in units of microns, is manufactured by these techniques.
However, the tolerance of products produced by these manufacturing methods is one-tenth of several microns. Thus, although a structure constituted by a micromachine is produced by these manufacturing methods, the necessary surface accuracy of a microstructure, which is less than the tolerance thereof, cannot be necessarily ensured. Moreover, it is difficult to assure the positional relationship among a plurality of switching elements with precision of one-tenth of several microns or less. Furthermore, the photolithography techniques are the most advanced techniques of processing microstructures, and those for processing silicon employed as a material. Thus, even in the case of using the photolithography techniques, it is difficult to process other materials, for example, a transmissive resin material suitable for an optical element.
Similar problems occur in other micromachines, such as an optical micro-switch for switching on and off signals transmitted between optical fibers, and a microvalve for allowing fluids to flow therethrough and shutting off the fluids. Thus, it becomes necessary to develop a micromachine, which can ensure the surface accuracy of microstructures and establish the positional relationship between adjoining ones thereof and provide a gap or a step therebetween with high accuracy, and a manufacturing method therefor.
Accordingly, an object of the present invention is to provide a micromachine, which can highly accurately manufacture a machine, whose size is in the order of a micron or less, for driving a microstructure, such as a switching portion, and a manufacturing method therefor. Particularly, an object of the present invention is to provide a micromachine suitable for a spatial light modulator having a device operative to modulate light by moving and controlling a microswitch portion, such as the optical switching portion utilizing evanescent light, which has a planar component, that is, a micromachine in which a high-accuracy surface is formed and in which a drive system for microstructures, and to provide a manufacturing method therefor.
Further, another object of the present invention is to provide a configuration, by which a micromachine having a drive system can be manufactured, and to provide a manufacturing method therefor.
Moreover, another object of the present invention is to realize a structure of a micromachine efficiently providing a configuration, in which a plurality of units each having a high-accuracy surface of a microstructure and a drive system for the microstructure are arranged in a array, with high precision, and to provide a manufacturing method therefor.
Therefore, according to the present invention, the techniques of a mold transfer method employed as a method for manufacturing microlenses or microprisms are used for manufacturing a micromachine having a drive system. However, in the case of using the mold transfer method, the surface accuracy of the microstructure can be ensured, while the drive system itself including a spring or a film cannot be manufactured in the micromachine. Thus, according to the present invention, a micromachine, in which a first microstructured portion drives a second microstructured portion of a predetermined shape, is manufactured by using a molding step, at which the second microstructured portion is manufactured by being put on the first microstructured portion and by forming at least a part thereof by the mold transfer method after the first microstructured portion serving as a driver system is manufactured. This manufacturing method can provide a micromachine that has a second microstructured portion, at least a part of which is formed by the mold transfer, and a first microstructured portion for driving this second microstructured portion.
According to the micromachine of the present invention and the manufacturing method therefor, a second microstructured portion, which is required to have surface accuracy in addition to a microstructure, can be efficiently manufactured with high precision by employing the mold transfer suitable for the second microstructure, instead of manufacturing the entire micromachine by one of the techniques. Moreover, as long as the mold transfer is employed, there is a little limit to the material. Thus, a transmissive microstructure, which is required by the spatial light modulator, can be easily formed in the micromachine. On the other hand, the first microstructured portion realizing a drive system, which can be constituted by a complex combination of a spring, a film, and a post, is formed on a silicon substrate by performing processes such as a silicon process which consists of a photolithography step and one of or a combination of a dry etching step, a wet etching step, an oxide film forming step, a crystal growth step, and a CMP planarization step, a laser processing process, a sol-gel forming process, a sintering process, and a machine cutting process. The first microstructured portion includes a structure caused by external power to work, an actuator caused by external energy to work, and a sensor type actuator varying in response to a change in ambient environment.
The microstructured portion, which is caused by external power to work, includes a gear, a hinge, and a plate spring. Further, the actuator, which is caused by external energy to work, includes an electrostatic actuator formed by the photolithography techniques employed in the silicon process, a piezoactuator formed by the sol-gel method, a piezo thin film actuator formed by a sputter method, a laminated piezoactuator, a heating actuator adapted to heat a sealed fluid by using a heater and to utilize a force generated owing to thermal expansion of the fluid, and a shape memory alloy actuator that comes to have an actuator function by setting a combination of shape memory alloys and individually heating the shape memory alloys. Furthermore, the element caused by a change in the ambient environment to work includes a bimetal actuator that is formed by putting metals, whose expansion coefficients differ from each other, together and deforms owing to thermal expansion of the metals caused in response to a change in ambient temperature.
The microstructures can be reproduced with good accuracy by forming the second microstructured portion by the mold transfer. Particularly, the flatness of the surface thereof can be ensured. Moreover, the positional relationship between adjacent members can be easily controlled with extremely high precision. Therefore, the second microstructured portion is suitably for the switching-on and switching-off of light and a fluid, namely, for realizing the switching function. Furthermore, according to the manufacturing method of the present invention, the flatness of the surface can be ensured with extremely high precision. Thus, the manufacturing method of the present invention is suitable for imparting the optical element function to the second microstructured portion. Therefore, a micromachine manufactured by the manufacturing method of the present invention is suitable for realizing a spatial light modulator of the size represented in the order of microns, in which light is modulated by the second microstructured portion.
Further, the microstructure can be reproduced with good precision by forming the second microstructured portion by the mold transfer. Moreover, the positional relationship between the members can be determined with extremely high accuracy. Thus, the manufacturing method of the present invention is suitable for manufacturing a micromachine by manufacturing a plurality of second microstructured portions by using the same mold so that the plurality of second microstructured portions are arranged in a two-dimensional array. Furthermore, in the case of a micromachine having a third microstructured portion, such as a post, which is not driven by the first microstructured portion, the positional relationship among these microstructured portions can be easily controlled with very high accuracy by performing the transfer of the second and third microstructured portions by using the same mold. Thus, in the case of a micromachine of the size in the order of sub-microns, which requires the step formed between the second and third microstructured portions and the gap formed therebetween, the dimensions of the step and gap can be stably reproduced with extremely high precision.
Although the entire second microstructured portion can be formed by the mold transfer, the effects of the present invention can be obtained by forming at least a part, which requires the surface accuracy and the positional accuracy thereof by the mold transfer. It is the same with the case of performing the transfer of a plurality of second microstructured portions or the second and third microstructured portions by using the same mold. Further, the mold to be transferred can be used a plurality of times. Thus, as compared with the case of producing micromachines, such as a spatial light modulator, by the machine cutting or the silicon process, the micromachines of the complex shapes can be easily produced. In addition, the production cost thereof can be considerably reduced.
As the first microstructured portion serving as the drive system, an electrostatic actuator, which can be controlled by electric power, is specially useful as, for instance, a spatial light demodulator. Therefore, the photolithography techniques employed in the silicon process, by which minute films and electrodes can be easily produced in the micromachine, is very effective as a method for manufacturing the first microstructured portion. On the other hand, there is the possibility that the first microstructured portion produced in the micromachine by performing this process is broken by stress generated when the second microstructure is formed on the first microstructure by the mold transfer. Therefore, it is preferable that first, the second microstructured portion is molded without etching a sacrifice layer provided in the periphery of the first microstructured portion, and subsequently, the etching of the sacrifice layer is performed. Furthermore, even in the case that the first microstructured portion is not formed by using the photolithography techniques, a sacrifice layer is formed around the first microstructured portion by applying resin therearound so as to prevent the first microstructure from being broken. Alternatively, it is preferable that a sacrifice layer made of SiO2 and aluminum by, for example, a plasma CVD method.
When the second microstructured portion is manufactured by the mold transfer, stress applied to the first microstructure can be reduced by fixing the first microstructured portion, which serves as a moving part, by using the sacrifice layer in this way. Simultaneously, the accuracy of the positioning of the first and second microstructured portions is improved by planarization of the first microstructured portion and the sacrifice layer provided therearound. Furthermore, mechanical polishing and mechanical-chemical polishing techniques are effective for the planarization at that time.
Further, when the second microstructure is formed by the mold transfer, a large amount of resin is used on the connecting surface between the first and second microstructured portions, namely, the boundary surface of the first microstructured portion. Thus, it is preferable for enhancing the connectivity therebetween that no metallic film is formed on such a surface. In the case that the first microstructured portion has a metallic film in the range in which the second microstructured portion is transferred, it is preferable that a non-metallic film made of, for example, silicon is provided in the range thereof. When the conjunction between the metallic layer constituting the first microstructured portion and the second microstructured portion is needed, it is necessary for enhancing the adhesion therebetween to rough the surface or to realize a wedge-shaped contact surface.
Methods for forming the mold, which is used at the time of forming the second microstructured portion by the mold transfer, include a method of forming the mold by precision machining, a method of forming the mold on a silicon substrate by the photolithography techniques, a method of molding (namely, performing the optical molding of) the mold by directly exposing a resist, a method of forming the mold by laser processing, and a method of forming and shaping the mold by electric discharge machining. Further, a mold capable of transmitting light can be formed by using a material adapted to transmit light instead of performing a plurality of steps.
First, a mold, by which a microstructure is formed with good accuracy, can be made by performing a combination of anisotropic etching and isotropic etching. For instance, etching is performed on a silicon substrate in KOH solution by adjusting an orientation plane of crystals and a mask to the (1,1,0) plane. Thus, the anisotropic etching progresses, so that an inverse triangle hole, whose vertex angle is 109.48 degrees, is formed therein. When the depth of this inverse triangle reaches a predetermined depth, the isotropic etching is performed by using SF6, which is a fluoride gas, so that a structure extending in a vertical direction is formed. Consequently, the mold for forming the second microstructured portion is formed. Moreover, a mold for simultaneously forming the third microstructured portion can be obtained.
Furthermore, it is effective to perform the mold transfer by applying a photosetting resin, such as an ultraviolet-set resin, onto the first microstructured portion, and using a transmissive mold. Further, the positioning of the microstructured portion can be performed according to the optical information processing by utilizing an alignment mark provided on the base of the first microstructured portion, and an alignment mark provided on the mold for forming the second microstructure. The positional relationship between the first and second microstructured portions can be limited with good accuracy. To adjust the relationship in position in the direction of height between the first and second microstructured portions, bubbles are efficiently removed from the resin or the portion between the resin and the mold by applying a photosetting resin on the first microstructured portion, and then bringing this portion, on which the mold is put, into a chamber, and subsequently, setting the ambient pressure of the photosetting resin, which is to be molded, at a value which is lower than the atmospheric pressure, before the resin is photo-cured, or evacuating the chamber in such a way as to produce a vacuum therein. Further, because of the use of the transmissive mold, the photosetting resin is cured by irradiating the photosetting resin with light (namely, ultraviolet) in such a state. Consequently, the second microstructured portion can be manufactured with high precision in high yield.
The transmissive mold can be manufactured by machining a glass material or a plastic member, which transmits light. When portions of more minute shapes are manufactured with high accuracy, a primary mold for forming the second structure portion is formed on an opaque silicon base by etching. Then, a secondary mold is produced by transferring the primary mold onto a transparent resin. Further, the photosetting resin can be irradiated with light radiated from the outside by the mold transfer using this transmissive secondary mold. As described above, it is desirable that the structure for limiting the positional relationship between the first and second microstructured portions is provided in the secondary mold.
Ultraviolet-set resin may be used as a material transmitting light and constituting the secondary mold. (Although a thermosetting resin, an epoxy dry resin, a polycarbonate resin, a polyimide resin, and an ultraviolet-set resin can be used as the resin, a manufacturing method using an ultraviolet-set resin is described herein by way of example. Examples of the ultraviolet-set resin are radical polymerization resins, such as an unsaturated polyester resin, an acryl type or polyester acrylate resin, a polyester acrylate resin, a urethane acrylate resin, an epoxy acrylate resin, polyether acrylate resin, an acrylate resin of the type including an acryloyl group on the side chain thereof, a thiol-ene type polythiol acrylated drivative, and a polythiol spiroacetal resin, and cationic polymerization resins, such as an epoxy type or epoxy resin. These resins may be used.) Further, the secondary mold can be formed by applying an ultraviolet-set resin on the surface of a silicon substrate formed in such a way as to have a shape corresponding to the second microstructured portion by etching or corresponding to the second and third microstructured portions in the case of simultaneously transferring these microstructured portions. More preferably, putting a transparent plate material made of polycarbonate, glass, or polyimide on the ultraviolet-set resin at that time and then ultraviolet-curing the transparent plate material. Thus, the patterns of the silicon primary mold and the transmissive second mold can be easily produced.
In the case that the second microstructure is formed by the mold transfer, the predetermined shape and performance thereof can be obtained by performing the mold transfer once. However, a constituent part having high surface accuracy can be provided in the second microstructured portion by performing the mold transfer a plurality of times. For example, after a first part having a V-shaped groove of the second microstructured portion is formed in the first microstructured portion by the mold transfer, a surface of this part is used as a reflection surface. Thus, a metallic film made of gold, silver, or platinum is formed thereon by using means, such as sputtering, deposition, or crystal growth, in such a manner as to have a thickness of several hundreds xc3x85.
Furthermore, an ultraviolet-set resin is applied onto the V-shaped groove of the second microstructured portion and onto a region acting as a post portion that may be used as the third microstructured portion. Then, the planarized transparent member made of glass, polycarbonate, or acrylate is pressed thereagainst as the transfer mold. Consequently, the ultraviolet-set resin is cured by irradiating ultraviolet rays onto the planarized transparent member. Hence, the second microstructured portion having the V-shaped groove and the flat surface serving as the extraction surface can be manufactured. Thus, the second microstructured portion for switching, which has a surface for extracting the evanescent wave, and a reflection surface, can be manufactured. Consequently, a macromachine serving as a spatial light modulator using evanescent waves can be manufactured.
The positional relationship between the post and the switching portion can be determined with good accuracy by providing a predetermined step between the post, which serves as the third microstructured portion, and the switching portion, which serves as the second microstructured portion, in the transfer mold. Moreover, the step therebetween can be controlled with good precision. At that time, bubbles are removed from the resin by setting the ambient pressure at a value, which is lower than the atmospheric pressure, or evacuating so as to cause a vacuum state. Thus, as described above, a more flat microprism is produced. The switching accuracy is enhanced. Consequently, a stable switching operation and high contrast can be obtained.
Further, the second microstructured portion, from which light is extracted, is formed from the resin. Thus, as compared with the case of employing glass material, the hardness is low. Moreover, the flatness of the light extraction surface can be ensured by the mold transfer. This results in an increase in the adherence between the surface (namely, extraction surface) of the second microstructured portion and the surface of the light guiding portion adapted to guide light by performing the total reflection of the light. Thus, the micromachine of the present invention can provide a more stable switching operation and a high contrast spatial light modulator.
Further, the second microstructured portion can be formed by the combination of the mold transfer technique and the photolithography technique. For example, an unnecessary part of the second microstructured portion can be removed by using the photolithographic techniques, such as masking, wet etching, and dry etching. Furthermore, in the case that the second microstructured portion is made of resin, the unnecessary part can be removed by being irradiated with light emitted from an excimer laser. At that time, the sacrifice layer provided for fixing the first microstructured portion can be removed by etching.